I see RT as Microsoft getting in a lot of their early development and preparation for the ARM market, which is growing. Eventually we'll likely start seeing more ARM server, laptops, and desktops. When that happens Microsoft will have a good foundation with Windows already mostly ported over to ARM. RT is a play for the long game.

NT also had support for a while for the Power architecture. They ended up abandoning it, but it was for similar reasons... If Power and PowerPCs took off they wanted to be ready. Unlike Power that was mostly on servers and workstations, ARM is mostly used now in consumer tablets and phones, which is where we find RT and Windows Phone (that now uses NT as well).